T-TEST: Also fix the case where the correlation is slightly less than -1.0.

[pspp-builds.git] / src / language / stats / t-test.q

diff --git a/src/language/stats/t-test.q b/src/language/stats/t-test.q
index c69a6cdb5bc5adb552afedbc209030d6ada3b12b..ba7e9388a95274560589d30605ce2be3e0720718 100644 (file)
--- a/src/language/stats/t-test.q
+++ b/src/language/stats/t-test.q
@@ -1,5 +1,5 @@
 /* PSPP - a program for statistical analysis.
-   Copyright (C) 1997-9, 2000 Free Software Foundation, Inc.
+   Copyright (C) 1997-9, 2000, 2009 Free Software Foundation, Inc.
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -1432,9 +1432,13 @@ pscbox (const struct dictionary *dict)
 
       double df = pairs[i].n -2;
 
+      /* corr2 will mathematically always be in the range [0, 1.0].  Inaccurate
+         calculations sometimes cause it to be slightly greater than 1.0, so
+         force it into the correct range to avoid NaN from sqrt(). */
+      double corr2 = MIN (1.0, pow2 (pairs[i].correlation));
       double correlation_t =
        pairs[i].correlation * sqrt (df) /
-       sqrt (1 - pow2 (pairs[i].correlation));
+       sqrt (1 - corr2);
 
 
       /* row headings */